As commonly admitted, infrared detection, in a broad sense, that is, in a wavelength range from 0.75 micrometers to 1,000 micrometers, is a technological field submitted to specific issues. Indeed, any object emits in the infrared spectrum as soon as its temperature is greater than 0° K. Thus, when the environment of an infrared detector is not cooled down, the members surrounding the sensitive elements (substrates, connectors, packages, optical systems, etc.) emit a significant infrared radiation which adds to the radiation from the scene which is desired to be detected. Such a non-useful component may be very significant and sometimes form a signal greater than that which is desired to be measured. A dark signal, which for example results from the thermal generation of carriers in the case of an IR photodiode, or from Ohm's law in the case of a bolometer, adds thereto. Such a non-useful component is commonly called “thermal noise” or “common-mode”.
Accordingly, and unlike other types of detection, and particularly visible detection, it is necessary to provide structures and operations capable of efficiently managing this common mode. For this purpose, the first highly-sensitive infrared detectors used to be cooled down to very low temperatures, in the order of some hundred degrees Kelvin, or even of a few degrees Kelvin, to minimize the common mode.
There further exist two different classes of infrared detectors, that is, so-called “quantum” detectors and so-called “thermal” detectors, and particularly, for this last category, bolometric thermal detectors. As is also well known, the physical principles implemented by these two types of detection are fundamentally different and induce their own issues.
In the case of quantum detectors, a semiconductor is used to generate electron-hole pairs under the effect of photon absorption in the infrared spectrum, the charge carriers thus created being collected via electrodes, most often combined with a PN junction.
On the contrary, in the case of bolometric thermal detectors, an absorbing material selected for its ability to convert the power of the incident infrared flow into heat is used. This material, or a second material in contact with the first material, is also used to convert the generated heat into a variation of an electric characteristic, generally an electric resistance variation. The variation of the electric characteristic is then measured.
To improve the detector sensitivity, a specific bolometric detector architecture has been designed, that is, a detector which comprises an array of bolometric microplates suspended above a so-called “read” substrate by means of support and thermal insulation elements.
As known per se, such an architecture is specifically provided to thermally insulate the bolometric elements from the substrate. Thereby, a significant gain in sensitivity is obtained and, on the other hand, this architecture also enables to do away with the implementation of a cooling down to a very low temperature.
Although an architecture using suspended microplates has many advantages, and particularly the possibility of being used with no cooling down to very low temperatures, the presence of the bolometric microplate support elements does not provide a satisfactory filling rate by means of current manufacturing techniques, the filling rate being all the poorer as the microplate manufacturing is high.
Solutions have been developed to increase the filling rate. However, the latter imply making manufacturing methods more complex, which results in higher costs.
For example, document U.S. Pat. No. 6,094,127 describes a detector having three stacked stages with, in particular, a stage comprising an integrated circuit, a support stage, and an absorption stage. The absorption stage may thus occupy the entire surface of the detector, thus improving the efficiency. However, to electrically connect the absorption stage to the support stage, an electric interconnect element is interposed between the support and absorption stages. The electric interconnect element is formed of a conductive channel encased in a dielectric sheath. This results in a complex manufacturing process directly putting at stake the electric continuity from one stage to the other of the detector, which continuity is however a crucial element for the optimal operation of the detector. Further, the presence of the electric interconnect element in contact with the absorption stage may deteriorate the absorption quality and the detector sensitivity.
Further, to increase the production volume and/or to decrease manufacturing costs, collective manufacturing methods are usually implemented, several arrays of microplates being jointly manufactured on a same silicon wafer and then individualized, as for example described in documents U.S. Pat. No. 6,753,526 and U.S. Pat. No. 6,924,485.
Given that collective manufacturing methods are already used to manufacture arrays of microplates, collective manufacturing methods originating from microelectronics are also used to form detectors directly integrating a vacuum packaging for each microplate, as for example described in the last two above-mentioned documents. Such a packaging, commonly called “integrated tight micro-packaging”, is formed of a capsule formed above each microplate and bearing on the substrate on either side thereof and tightly vacuum-sealed. The collective carrying out of the packaging steps thus enables to decrease detector production times and costs as compared with a single tight package individually formed for each microplate array.
However, the space necessary between microplates for the capsule bearing translates, for an equal array size, as a significant decrease of the optically-active surface of the detector, and thus as a direct decrease in the detector efficiency.
Thus, by construction, the useful surface area of a bolometric microplate suspended by support elements, dedicated to the detection of an infrared or terahertz radiation is limited with respect to the substrate surface area, which decreases the detector sensitivity.
For example, the forming of detectors with square microplates having a 12-micrometer side length, which dimensions currently form the maximum degree of miniaturization of bolometric microplates, and absorbing around λ=10 μm, requires for each microplate a square substrate surface area having a side length of at least 17 micrometers. The useful surface area of an array of microplates having a 12-micrometer side length, dedicated to the detection, thus amounts to at most 50% of the total surface area of the array.